1. Field of the Invention
The present invention relates generally to battery separators and particularly to such separators for use, for example, in cells of high energy density secondary alkaline batteries.
2. Description of the Prior Art
An illustrative environment where application of the principles of the present invention is particularly advantageous is in high energy density batteries such as, for example, the batteries as described in U.S. Pat. No. 3,625,770 to Arrance et al dated Dec. 7, 1971.
The patent describes a battery separator designed for silver-zinc (Ag/Zn) alkaline electrolyte cells. The described separator includes a film coat which had been applied from a solvent system onto the outer surface of a fuel-cell grade asbestos substrate. In the cited patent, the film coat formulation comprises: (1) reactive and inert fillers, the reactive fillers being soluble in the alkaline electrolyte; (2) an organic polymer binder, e.g. polyphenylene oxide (PPO); and (3) a plasticizer. Volume resistance of this separator was considered very acceptable, e.g. 14 ohms-cm. This resistance was well below the normally acceptable maximum value for (Ag/Zn) cells of 60 ohms-cm. For further information related to U.S. Pat. No. 3,625,770, see NASA report TMX-3199 "Factors Influencing Flexibility, Resistivity, and Zinc Dendrite Penetration Rate of Inorganic Separators For Alkaline Batteries" which provides an analysis of the film composition. Another report, NASA TMX-3080, "Structure and Function of an Inorganic-Organic Separator For Electrochemical Cells--Preliminary Study" describes the structure and function of the patented separator.
When the (Ag/Zn) separator of the Arrance et al patent was evaluated for use in nickel-zinc (Ni/Zn) cells, it was found that because of the brittle character of the separator, the film coats cracked during handling. Therefore, there appeared to be a need for a more flexible separator.
U.S. Pat. No. 4,085,241, dated Apr. 18, 1978 to Sheibley, describes a new separator having basically the same film coat formulation as described by the Arrance et al patent except that the film coat is more flexible. In the latter patent, the solvent based PPO binder was replaced by a thermoplastic elastomer copolymer. This latter separator also provides acceptable volume resistivity values, e.g. less than 25 ohms-cm. NASA TMX-3465 entitled "New Separators for Nickel-Zinc Batteries" provides further information concerning this flexible separator.
Recognizing possible environmental risks in employing asbestos as a substrate material for battery separators, as well as the added risk of using solvents in the preparation of the coating formulations, the present inventor, George Schmidt, along with Robert Weber, developed aqueous-based substitutes for rubber-binder film coat composition taught in the Sheibley patent. Pending patent application Ser. No. 001,924 to Schmidt and Weber "Flexible Separators for Alkaline Batteries" filed Jan. 8, 1979 discloses the aqueous based substitutes, as well as a new separator material in which the film coat is applied from an aqueous system. The separator may include a meltblown thermoplastic polymer web to replace the asbestos as the separator substrate.
When separator materials of this nature are manufactured, sheeted and stored in layers, undesirable sticking of one sheet to another was observed. This is believed to be due to the inherent tackiness of the binder and the softening effect of the monomeric plasticizers. To resolve this problem, a release liner had to be placed between layers. A search for various other means to avoid the sticking problem was initiated. This search was successful and resulted in the present invention.
The present invention is directed to the improved product which resulted from confronting and solving the basic sticking problem as described above. In the course of the development, additional unforeseen problems were also solved as will be apparent.